The present invention relates generally to self-tapping or thread forming screws, and more specifically to such screws which are designed for engagement or insertion into relatively hard plastic material.
Thread forming screws are known for use with a wide variety of materials, including sheet metal and plastics. Typically, the screws of this type which are designed for use in plastics are configured for use in relatively soft thermoplastic materials, which are characterized by the property of deforming a certain extent into the gap between adjacent thread ridges. Such screws typically have a relatively high profile thread, or a large major to minor diameter differential when the diameter of the threads is compared with the diameter of the core or shank. This differential is intended to prevent the flow of displaced plastic into the area around the core.
Another design criteria of such screws is to increase the amount of loosening torque required to remove the screw, to prevent unwanted unthreading. One approach is to make the thread of the screw asymmetrical, with the threads having a straight edge on one side and a sharp bend on the other side. International Application No. 99/06715 (PCT/EP98/04719) discloses a variety of thread configurations in which, instead of being straight, at least one of the thread edges has a designated angle formed in the edge. The variety of configurations is intended to provide threads which are more suitable for various types of plastics, and the angle formed on each edge is intended to provide a space closer to the screw core into which displaced plastic may flow when the screw is turned in the plastic material.
International application No. 99/06719 (PCT/EP98/04720) discloses a thread forming screw with a flank angle of 30xc2x0 and a relatively thick core for the diameter of the thread ridges. In addition, the thread ridges are relatively widely spaced. In that application, the thread surfaces are straight, i.e., not angled.
It has been found that when screws designed for use with relatively soft plastics, in which the soft plastic material is displaced toward the core of the screw, are employed in relatively hard plastics such as thermoplastics, including glass-filled, talc-filled or other types of thermoplastics as well as equivalently hard engineered materials, the screws have a tendency to crack the plastic into which they are being threaded. While standard sheet metal screws have a thread flank angle of approximately 60xc2x0, this size flank angle has been found to contribute to the cracking problem. As exemplified by U.S. Pat. No. 4,544,313, thread forming screws for use in plastics have been disclosed with flank angles in the range of 30xc2x0 to reduce stress on the plastic. However, these are generally disclosed with symmetrical thread configurations, i.e., symmetrical about a line perpendicular to the axis of the core which passes through the apex of the thread.
As also disclosed in U.S. Pat. No. 6,113,331 by decreasing the flank angle, more threads per inch may be provided, which increases the required tear-out force. A high tear-out force requires a stronger screw, and this property is typically achieved by increasing the diameter of the core or using costlier, highly tempered materials. However, large diameter cores have not always been considered suitable for use in plastics, since relatively small diameter cores have been thought to provide a larger area for receiving displaced softer plastics.
Accordingly, it is a first object of the present invention to provide an improved thread forming screw which is designed for use in relatively hard plastics but will not cause cracking of the plastic upon insertion.
Another object of the present invention is to provide an improved thread forming screw which is designed to have increased resistance to pullout.
The above-listed objects are met or exceeded by the present high profile thread forming screw, which features an asymmetric thread profile forming a flank angle of approximately 30xc2x0 which reduces the tendency to crack relatively hard plastics. Another feature of the present fastener is that it has a relatively small pitch compared to conventional fasteners, which allows more threads per inch, increasing pullout resistance. In addition, the core diameter is sufficiently large to provide the necessary strength to withstand the increased pullout resistance.
More specifically, the present invention provides a thread forming screw configured for insertion into relatively hard plastics and includes an elongated core region having two ends, a head at one end and a point at the opposite end. A helical thread extends generally radially from the core region, the thread being asymmetrical when viewed in a cross-section taken through a diameter of the core, and defining a flank angle made up of a leading angle, facing the point, and a trailing angle facing the head. The leading angle is greater than the trailing angle.
In the preferred embodiment, the leading angle is approximately 20xc2x0 and the trailing angle is approximately 10xc2x0. In addition, the thread has an outside diameter and the core has an outside diameter, and a ratio of the outside diameter of the thread to the outside diameter of the core is approximately 1.30:1. The thread, specific to a M5.0 screw, has a pitch range of approximately 1.15 mm to 1.35 mm, which is reduced from conventional screws having pitch ranges on the order of 2.25 mm.